Ok, lets stay logical.Disconnect the battery.Put the wire on the sender to ground.Measure at the connector the wire that goes to the sender and ground.It has to be lower as 5 ohm.If it is much higher you have a bad conection in the wire to the back.

YOu have found the problem. There is a short circuit, most likely a melted wire somewhere. Now you "just" need to trace it down!

But isn't there a problem with calling this a short curcuit....? if there was a short curcuit, the wire would be grounded eg. 0 resistance, which would make the meter go to the top.... and the problem here is no movement, which would suggest no connection, but the meter went sky high when I grounded it at the sender when the dashboard was connected....

The circuit for temp is simple, and there is not much that can go wrong.One side of the gauge is + the other side is going to the sender.If you measure the wire to the back the way I described the + on the gauge and that the engine at the point the sender is, is realy grounded it has to work.If not, only sender or gauge can be defect, and I suspect the sender.

The way you work with electrics is not right in the first place I think..(no offence).If you measure ohms in circuit always disconnect battery.If connected measuring can be different.I am hammering on the fact the engine has good ground because the chassis is galvanised and if connections on the chassis are coroded the protection layer of the chassis will try to correct that crawling unther the connections to correct rust making the resitance of connections higher.Measure it from minus accu to engine (it has to be around 3-5 ohm).Measure the wire as described reolace sender or/and gauge.This must do the job.

Edit.

and another thing, just put on a new connecter at the sender.The wire gets hot and at that point after so many years the connection is not reliable.I had that on my Dynamo wich didn't charge sometimes.All looked fine but it was rotten just behind the connector.

that's far too much! It should be very low, less than the resistance of the test wires to your ohm meter.

About the tab on the sender - yes it may be a good idea to replace it, but a visual inspection should be able to show whether there's a problem. This particular place is hot and dry, so it's not the worst place for corrosion to build up.

The worst places are the three connectors in the front compartment and the green connector in the engine room. These are open and located in wetty places. Partly melted connector housings are a typical indication of this, but only happens on connections that carry current, of course - which the temp sender doesnt, however.

I saw this problem this weekend on a friends' Espace, where a couple of fuses had melted down completely because of corrosion on the connections!

The best way to prevent it is to make all connections water proof. Silicone grease is quite useful for this.

that's far too much! It should be very low, less than the resistance of the test wires to your ohm meter.

About the tab on the sender - yes it may be a good idea to replace it, but a visual inspection should be able to show whether there's a problem. This particular place is hot and dry, so it's not the worst place for corrosion to build up.

The worst places are the three connectors in the front compartment and the green connector in the engine room. These are open and located in wetty places. Partly melted connector housings are a typical indication of this, but only happens on connections that carry current, of course - which the temp sender doesnt, however.

I saw this problem this weekend on a friends' Espace, where a couple of fuses had melted down completely because of corrosion on the connections!

The best way to prevent it is to make all connections water proof. Silicone grease is quite useful for this.

The calculation of the wires were included in the measuring, and if you measure from the front the wire is very long, so thats why I held a higher resistance.The thick wire from front to end has to be almost zero.

Closing connectors with silicongrease is a good idea, but do it with acid free kit.The acid in some kit can destroy cupper over time.

The heat in the connectors at the engine can make that points very unreliable over time, as I already found out.The problem I had on the dynamo took alot of time to find it out, because it was there some times.Better to replace connectors if you are there doing jobs.

BTW, does anybody know the location of the headlamp relays? sine some previous owner have something of his own, and it doesn't look good, I thought I might see if the orginal fuses are gone, beyond repair, or just have bad wiring since he decided to do what he did.